Popular Science Monthly/Volume 7/May 1875/Modern Street-Pavements

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THE most distinguished sanitarians of the age have established the fact that our modern cities are mostly so located that public health depends much less upon climate and position than upon rational conditions and modes of life. Enforced cleanliness, and the progress of sanitary works in cities, are followed by an enhanced vitality and elasticity of mind still more than by longevity of the inhabitants. Among sanitary works, improved pavements are classed along with sewerage, water-closets, and water-supply under pressure; since it is a prime condition of public hygiene that every street and alley should drain as promptly and thoroughly as the houses erected on it. A proper observance of these maxims has materially contributed to the reduction of the annual death-rate of London within the last two centuries from forty-two to twenty-two per thousand, notwithstanding the unprecedented increase in the numbers and density of the population. An average decrease of thirteen per cent, in the death-rate has been traced directly to the influence of modern sanitary works, introduced into cities mostly during the last twenty years; and a thorough reform in pavements must give still more striking results.

The material for pavements is mostly decided upon by non-professional municipal authorities, and upon these an enlightened public opinion must exert a beneficial influence. A condensed review of the subject, in the light of history, technical science, hygiene, and finance, will help excite reflection, and to mature rational views, and will furnish a timely contribution to the literature of the day. Manufacturing industry, commerce, and railroads, those important motors of modern civilization, have combined to increase the number and size, and to concentrate the internal traffic of large cities, so that horses and vehicles have steadily increased, absolutely as well as in proportion to the population. Under the same influences an enormous wealth, formerly unknown, has been amassed in the cities, and whole streets have risen, lined by majestic buildings, in uninterrupted succession; while, even in the older or less pretentious streets, houses of a mere utilitarian character disappear, to make way for structures with an elevated standard of architecture. What at an earlier epoch was the proud privilege of the famous capitals of Italy, the exceptional luxury of their dwellings and mansions, is now to be found in most modern cities, though the effect be not as overpowering, on account of a want of harmony in the style.

Simultaneously with the higher wants resulting from greater wealth and closer contact, whole cities have been transformed from loose aggregates of irregularly-scattered houses into well-organized systems, all the elements of which, though serving individual purposes, are intimately connected by the complicated net-works of pipes supplying fresh water, discharging waste water and soil, and furnishing light during the night to the streets as well as to the houses, from cellar to roof; to which, perhaps, the inventive genius of the age may add, before long, the supply of heat for domestic necessities and personal comfort. In such a complex organism, the roadways and sidewalks are not merely spaces set apart for light, air, and traffic, but they are component parts of the wonderful machinery devoted to these purposes, and bear close relations to the dwellings which they separate and connect, and the restorative veins of which they cover as a protecting crust.

Of all these coöperating agencies, the least attention, until recently, was paid to the construction of roadways. Cobble-stones were resorted to for paving-purposes, since they were easily obtainable in the alluvial plains in which most modern cities are founded. They were succeeded by irregular quarried stones of such quality as was within easy reach; then by larger square blocks, mainly of trap-rock or granite, such as were thought necessary in streets with heavy traffic. But experience has proved that the jarring against them compelled the construction of heavier wagons, and that their peculiar smoothness by wear caused the horses to fall, and so this material was modified to uniform oblong blocks in narrow courses. These, after severe tests, have maintained a truer surface, have been found to offer a greater resistance against wear, to lessen the noise, and to decrease considerably the number of accidents to horses. They are called, in common with the former, Belgian blocks.

A most important sanitary feature, almost entirely neglected before the rapid concentration of population in the cities, now demanded attention. The cubical stone blocks are displaced under the prodigious traffic, the corners and edges are worn away, the surface gets to be irregular, the joints are widened. The filth of the streets gathers in ruts and joints, is recruited constantly by new accessions of urine, horse-dung, and silt, and, diluted by the rain, it ferments, and forms a putrescent organic mire, becoming in course of time a source of noxious miasmas. In hot and dry weather these nauseating deposits pass into the atmosphere in the form of unhealthy vapors, or, pulverized and drifted by the wind, cause inconvenience and poison our lungs. Indeed, in repairing old pavements, a black layer of ground, saturated with sulphuretted hydrogen, is found below the stone blocks, and bears witness to the infection of the subsoil by the soakage of contaminated water. Prof. Tyndall has established by experiments that a large proportion of the particles of dust in the rooms of London houses is of organic origin, and other experiments have demonstrated that horse-manure, in a state of decomposition, is a permanent ingredient.

Vapors still more noxious than those from the road-bed of the streets rise from the gutters, the subsoil of which is saturated to a considerable depth by more concentrated matter of the described composition, and also from the surface of alleys on which are the houses of great numbers of people of limited means. Crowds of dirty children, whose tender lungs breathe the air immediately over this miasmatic soil, here contract constitutional predispositions, which doom them to a languishing and miserable life, and render them an easy prey to epidemics. This infection of the subsoil has been prevented, with a certain degree of success, by foundations of concrete. There is still another feature of stone pavements in the heart of cities, which affects the inner man more than the physical frame, viz., the rattling and noise, under heavy traffic, accompanied, in alluvial soil, by vibrations of the adjoining buildings. People with strong nerves, and accustomed to this rattle from early youth, may to some extent become hardened, but they will never get to be insensible to it; any indisposition is aggravated by the nuisance, and for recovery they hurry to the country. People with weak nerves, especially delicately-organized women, suffer great and permanent injury to the health. Nothing but the constant torment has partially dulled us to this evil. If cities had never been afflicted with this noise, and if, in a competition with other more suitable materials, stone pavements were adopted, a storm of opposition would soon sweep them out of existence again. Some of these difficulties have been obviated by using smaller and harder stones; but the objection to the improved Belgian pavement in general use, on account of the germs of disease stored in the wide joints and under the blocks, still remains.

To do away with the objection to stone pavements, efforts were made to introduce into cities the macadamized roads, which had proved eminently successful as country roads; these efforts have proved signal failures, though, when properly made, and in thoroughly good order, macadamized roads approach perhaps more nearly the desiderata than most others that have been tested, and are among the pleasantest and safest roadways in ordinary use. But the constant outlay for repairs, the difficulty of traction over them when recently laid, and considerations of hygiene and comfort, are such serious objections that they are gradually being displaced by other kinds of pavement. Whoever is doomed to live on a macadamized street needs no description of its horrors. These streets have justly been nicknamed crushing-mills for granite. Six hundred and fifty thousand tons of granite are annually pulverized on the streets of London, of which but one-sixth is due to the wear of paving-stones, the rest is attributable to the macadamized roads. This dust has to be scrubbed, washed, and brushed ever so often from clothes, furniture, stairs, and floors, before it is finally removed through silt-basins and carts, or sewers and river.

A little rain transforms these streets into broad slush-beds from which every thing within reach is bespattered by the hurrying wheels of vehicles. Ladies with modern garments cannot cross them, and whoever visits along such streets must leave a certain quantity of dirt on floors and carpets. But mud is not the worst affliction, for this mash, consisting of stone-dust, sand, and horse-dung, is transformed into dust by dry and hot weather, is whirled up by the rolling wheels, or, still worse, is drifted by wind, rendering the air unfit for respiration, penetrating into the tender, sensitive cavities of the lungs, settling on skin, hair, and clothes; suffocating the flowers and green leaves of plants along parked streets; forbidding the opening of windows, fouling the glass, and driving through the joints of the sash; lodging in curtains and blinds, spoiling the costly products of industry and maufacture in the show-windows, and haunting the anxious housewife in kitchen, pantry, and cellar. The devoted denizen finds but a partial protection against the shocking nuisance, when he mixes this dust with an abundance of water—by sprinkling the street.

A modification of the macadam is the Telford road; it consists of a bed of firmly-wedged quarry-stones, with an even surface as a foundation, upon which a layer of larger and a layer of smaller broken stone, mostly trap-rock, are spread, each being rolled by horse and steam rollers. Upon the well-compacted surface a binding of screened gravel is applied, moistened and rolled in, so as to present one solid mass, which, while hard and durable, yet retains some elasticity. This variety, superior for country-roads, though still open to the vital objection of dust, is equal in price to the costly modern city pavements, and therefore has found but a limited application within city limits—for instance, on the Boulevards of New York.

Wood pavements which, at one time, were much used in Britain, especially in London, and also in New York in 1835 and 1836, but were abandoned for weighty reasons, and especially on account of their rapid decay, were revived in the rising cities of the great West, notably Chicago, where stone was scarce, lumber was cheap, and a porous, sandy subsoil retarded the decay of the perishable woodblocks by dry rot from below, as happens on retentive soil. These wood pavements, smooth, noiseless, and advantageous for traction, were rather hastily adopted by municipal committees or boards in Eastern cities, where the conditions were different, and where decomposition commenced after two or three years' use. The heavy profits made induced a desperate fight in their favor by interested parties, a renewed effort in behalf of "treated" wood gave them a respite and a second harvest before final disuse, which was accelerated, however, by the overwhelming complaints of the offensive and unhealthy effluvia emitted from them; so that, in a sanitary point of view, the advantage of the absence of stone-dust was much overbalanced by the decomposition of the material itself.

The wood-blocks during treatment have been mostly impregnated, by pneumatic processes, with chloride of zinc, sulphate of iron, or oily, creosotic substances; and, though railroad-sleepers, telegraph-poles, etc., have been satisfactorily preserved through these agencies, such methods have failed, for various causes, to render an equivalent for the expenses incurred in treating the paving-blocks. But the patience of the people is not yet exhausted, and, in Northwestern cities, a new and costly revival is being arranged by the substitution of sulphate of copper for impregnation, a substance used in France, under M. Boucherie's patent, for thirty-five years past. The District of Columbia has been preëminently the experimental ground for treated wood pavements. An investment of about $5,000,000, a sum far in excess of that in any other city of the globe, has been made there within little more than three years in wood pavements, nearly all treated; many of them are now in an advanced state of decay, and, from the degree of preservation after two or three years' use on suburban streets with hardly any wear, one cannot approve of any of the processes applied, since none of them have effectively neutralized the destructive local agencies, or made up for inferiority in the quality of lumber used. Square, polygonal, wedge-shaped, and undressed round blocks, of pine, spruce, and juniper wood, set in rows, interlocking or parted by interstices, upon sand, board, or concrete foundations, were tried, so that all classes of patentees had chances to trot out their hobbies and gratify their passion to serve the community. Though this is an interesting study, we cannot in this place do full justice by entering into details.

The idea of ranking expensive wood pavements, treated and untreated, as valuable standard pavements, where more substantial materials can be procured at the same or lower prices, will, before long, hardly more than elicit a smile from the critical expert.

In this state of the problem, it may be considered as a new epoch in city-life that the increased facilities of commercial intercourse, by cheapening the cost of transportation, have brought a relief within reach, namely, asphaltum for roadways. The nature of asphaltum is frequently misunderstood, because the mineralogist, in speaking of asphaltum, has reference to the brittle bitumen usually found in Nature, while the civil-engineer designates by mineral asphaltum a porous limestone, in combination with tough bitumen. This limestone was primarily impregnated by volcanic action with petroleum, which appears to have oxidized within the structure of the stone by the slow action of many centuries. Thus both ingredients have been united so thoroughly in the asphaltum that neither heat nor water, nor the combined action of both, in causing decay, can render it hard and brittle by abstracting the tough bitumen from the limestone. It is not strange that the efforts artificially to imitate this intimate union have often produced materials with quite different powers of resistance against the various destructive agencies and vicissitudes of climates; and that the lack of durability—not to speak of numerous dead failures in various compounds of raw or treated coal-tar and coal-pitch with coal-ashes, saw-dust, cinders, sand, gravel, etc., commonly called concrete pavements, which have reduced moderately dirty streets, under the influence of the heat of summer, to vast sticky quagmires—has formed a serious drawback to the introduction of better material, and especially the well-tested native asphaltum, which will probably, in our climate, many times outlast the best artificial composition yet known, as it has done in other countries. The admixtures and distillations from coal-tar and pitch have been amply relied upon as the base for artificial concrete, on account of a supposed resemblance to the native asphaltum. This idea, however, is not sustained by modern scientific tests. Seen in thin layers under the microscope, the bitumen, the color of which is otherwise a deep black, shows a transparent yellowish mass, while coal-pitch is visible as a mass of coherent black points on an orange-colored ground. This investigation of the mastic relied upon suffices to explain in the one case the quality of toughness and binding power, and in the other that of brittleness.

Efforts are now being made to produce concrete pavements based on mixtures of silicate of soda with Portland cement. The latter, along with native asphaltum, is undoubtedly the most important modern building-material, but it has its separate province, and lacks just those qualities of the native asphaltum which are so highly appreciated for paving-purposes. It will hardly ever be successful in the long-run when encroaching on the sphere of the competing material which it has fairly outrivaled as a cement for brickwork and masonry, for which, in ancient times, asphaltum enjoyed a just celebrity, as attested by the remnants of the walls of Babylon and Nineveh. This class of pavement has been tested carefully in France with the well-known béton coignet and has failed. While the artificial mixtures soon require expensive surfacing, the native asphaltum when taken up for piping, or otherwise, after many years' wear, may be used again by simply heating and treating it as at first. In this aspect it bears such a relation to the artificial concretes as a copper roof does to a common tin roof.

Nature has unfortunately produced this valuable mineral deposit in but very few cases, and it has not yet been found in America, for the so-called Trinidad asphaltum consists mainly of bituminous scoriae, cemented together with vitrified sand and earth; and even the more esteemed Cuban asphalts contain from 27 to 34 per cent, of earthy substances. The deposits from Tyrimont-Seyssel, on the banks of the Rhone, in France, were the first to be used for pavements. But, as they contain only from 6 to 8 per cent, of bitumen, the powdered rock was found rather too dry, and therefore was superseded by the extensive deposits of the Val de Travers, the most important valley debouching from the Jurassic mountains of Switzerland into the Lake of Neufchâtel. These, with steady march, have conquered the markets of the world. The deposits known as Neufchâtel rock contain, with a constancy not found anywhere else, from 11 to 12 per cent, of bitumen—a most favorable proportion. Besides, they have absolutely greater toughness as a result of their degree of oxidation. They were formerly extensively used as a mastic for sidewalks, and form an excellent material for carriage-ways. They have been used since 1854 in Paris, and since 1868 in the principal thoroughfares of London and other European capitals.

The success of this bituminous rock pavement is by no means due to the lucky hit of one individual; it is the legitimate result of the persistent efforts of some of the best engineers of the age, by which all obstacles have been gradually overcome. The first trials were made with mastic, consisting of the powdered rock melted with mineral tar as a flux, and mixed with sea-grit which was laid upon an ordinary concrete foundation; they were followed by experiments with mastic, cast into blocks at the workshops, and laid with wide joints, which were filled in again with heated mastic. Next we hear of tests with broken asphaltic rock, rammed in a cold state upon a macadam foundation. And finally these intelligent labors were crowned by the splendid improvement of the compressed rock pavement, for which the rock, reduced by heat to powder and rammed and rolled while yet hot, into a homogeneous, tight covering, is laid upon a perfectly dry ordinary concrete foundation composed of crushed stone and cement. This simple improvement virtually adapts the old principle of a barn-floor of rammed clay, for thrashing, to the requirements of the open air, by making it water-proof. In place of the mastic, which attains consistency, by the congelation of the melted mass, without application of pressure, this "merely compressed body, in which the molecules of bitumen and limestone are soldered together by heat and ramming," obviates all tendency toward brittleness, without in the least interfering with the advantages of perfect homogeneity or water-proof qualities. It stands to reason that the mastic, which, notwithstanding its mixture with grit, is more or less pitchy, would be surpassed in elasticity and pliability by a merely kneaded mass. These pavements are reported to have withstood the extreme heat of Bombay, Hindostan, as well as the greatest known cold. Not affording any escape to the terrestrial heat through joints, they are kept warm and open from below in most cases when block-pavements present an icy surface. Their smooth, seamless face, being almost entirely free from abrasion by attrition or atmospheric action, meets the mechanical and hygienic objections to block-pavements, both of stone and wood, as well as of macadamized roads.

The asphaltum pavement is clean and fit for traffic a few hours after being laid, while new or repaved stone roadways must be covered for months with heavy layers of sand, to be drifted by the breeze in dry weather and added to the mud in rainy spells. Repairs can be made to the asphaltum pavement in dry days of a cold winter, while with stone pavements any defects must be endured until spring. Besides the sanitary advantages, the saving in temper, clothes, shoes, and furniture, is not to be overlooked. The popularity of this pavement in the two largest cities of Europe, where, with immense traffic and most extensive experience on the relative value of pavements, the demands on the municipal authorities are inexorable, serves as a proof that smoothness of surface does not cause any danger with this material. Being elastic but not soft in summer, and hard but not brittle in winter, it possesses with a slight yield the power of readjustment in a high degree, so that horses and drivers ever seek it, if it is "laid intelligently by practised bands, with a low crown" or flat profile.

A low crown is practicable because there are no surface-obstacles to drainage. It is also needful to prevent horses from falling on any spaces with a heavy decline toward the gutter. What is essential for the transverse profile of these streets is no less essential for their longitudinal profile: they must have easy grades—say a pitch of less than two per cent.—since the momentum of inertia of the masses in motion enters the problem. The smooth surface intensifies the downward motion of the wheels while decreasing the friction of the hoofs of the horse, which furnishes the power of resistance against the downward motion of the vehicle, or serves for affixing the power necessary to move the vehicle up-hill. With these precautions relief is afforded to the horse, this faithful companion of man, which, being dumb, is so often brutally ill-treated and abused.

It would lead too far to enlarge upon the numerous official experiments and observations made in Paris and London by which, though made under circumstances most unfavorable for the new pavements, the proportion of accidents to horses and vehicles has been shown to be considerably less on the asphalt than on stone pavements, except in the rare case of a muddy street during wet weather. Ordinary care can achieve much, in that direction. When driving on to an isolated asphaltum road, to which, in wet weather, mud has been dragged from adjoining streets of old construction, the change ought to be managed by checking the horses and gradually returning to full speed. As, by degrees, the regeneration of the streets becomes general, this temporary precaution will become unnecessary. Allusion to these minor details was deemed à propos, since the human mind is so constituted that little is generally thought of accidents of daily occurrence, while we are apt to be severe and even unjust against novel improvements, which, of course, in the beginning present more or less difficulties to be overcome under actual tests. The same man who unconcernedly sees a horse fall on a stone-paved street, or blames the driver, and even the horse itself, regardless of the pavement, might be loud with complaints or fears about the falling of a horse if traveling on a road of new construction.

During the careful examinations as to the merits of the new pavement, questions were raised regarding its fire-proof qualities. Indeed, we hear that, during the siege of Paris by the Germans, the population, visited by cold, and short of fuel, tore up the asphaltum roads to enjoy fires fed therewith. But, on the other hand, it is also recorded that, during the eventful time of the Commune, when incendiarism was frequent and ingenious, these pavements never caught or spread the fire, the proportion of the combustibles to the incombustibles in the asphaltum of the streets being too small to feed the fire. The matter-of-fact people of London were not satisfied with any thing less thaw severe direct tests: their engineers had to pile wood upon the asphaltum pavement, pour petroleum over it, and light it. When the fire burned most lively, and there were plenty of red-hot coals underneath, the space was cleared and nothing but little flames were noticed, which immediately died out. G. A. Shaw, the head of the London fire-brigade, who attended, declared expressly his faith in the harmless nature of the pavements during conflagrations. There is no doubt that asphaltum pavements may occasionally fail, but, when they do, this is attributable not to the material, if unadulterated, but rather to the method of its application, which requires skilled workmen, whose eyes and hands are quick and directed by an intelligent mind. The District of Columbia has about $1,750,000 invested in concrete and asphalt pavements, including the various patented mixtures and the natural asphaltic rock, and, though a certain degree of success has been attained under some of the patents, this does not appear to be uniform and under control of the engineers; nearly all show clear evidences of disintegration, and are periodically in need of repairs or resurfacing, which latter means virtually a failure of the patented process, while the pavements of natural rock rather improve by wear, and their first cost, in depreciated securities, was but $4.25 per square yard against $3.20 for the patented admixtures.

After these explanations, based on personal observations, as well as on the results of the experience of the leading engineers in this branch, the conclusion may well be drawn that asphaltum roads are destined to be the city pavements of the future—a destiny which is determined by the progressive spirit of the age, and which cannot be retarded for any length of time; it involves the interests of all, both high and low. If the most elegant and most frequented streets have the privilege to lead the van, it ought to be appreciated that the luxurious life of the higher classes depends upon the strength and activity of the children of the industrious classes as much as upon the toil of the farm-hand who, fortunately enough, is enabled to recruit his strength in open fields; hence, justice should be done likewise to the demands of health for the poorer classes, who, in consequence of the highly improper laying out of the cities, as bequests of by-gone generations, are frequently doomed to live in alleys and lanes, and these should be drawn into the vortex of a reform which, when accomplished, will gladden the humanitarian, whose head and heart are in sympathy with civilization in its noblest aspect.

Washington, March 26, 1875.